1. Field of the Invention.
The present invention relates to polymeric articles and more particularly relates to polymeric materials of high clarity and resistance to sterilizing radiation and method of preparing same.
2. Background of the Invention.
Semicrystalline polymeric materials, including the polyolefins of which polypropylene is most significant with respect to the present invention, are often employed in producing articles subsequently subjected to irradiation sterilization techniques. For example, in the health and medical field, these sterilizable articles include syringes, tubing and tube assemblies, microbiological plastics, flasks, package film and the like. It is well known that these semicrystalline polymeric materials, if not properly stabilized, will discolor and become embrittled as a result of sterilization by exposure to high energy radiation at levels above 0.1 megarads.
Further, after irradiation has been completed, post irradiative oxidation continues due to free radicals generated by the irradiation which participate in branching chain reactions. Therefore, while degradation of the mechanical properties of these polymeric materials may not be obvious immediately after irradiation, they become more pronounced as time goes on. Thus, much effort has been expended toward agents or additives which would stabilize polymeric materials toward post irradiation degradation.
Some recent attempts have been made to improve the stability of semicrystalline polymeric materials so as to reduce embrittlement. U.S. Pat. No. 4,110,185 discloses flexible sterilized articles comprising a semicrystalline polymer having a noncrystalline mobilizing additive incorporated therein. The additive increases the free volume of the polymer which is believed to account for the improved stability during and subsequent to irradiation.
In U.S. Pat. No. 4,274,932, a further improvement in radiation stability is achieved by including the above mobilizing additive in a semicrystalline polymer having a narrow molecular weight distribution.
U.S. Pat. No. 4,563,259 to Rayner discloses a substantially crystalline polyolefin of defined molecular weight distribution containing a sterically hindered amine stabilizing agent.
In some applications, as, for example in the packaging industry, polymeric materials are used in forms such as plates, sheets, films and the like in which clarity or transparency is a very desirable property. Clarity may also be important for certain plastic articles, such as syringes, made by injection molding.
In general, clarity is not an inherent property of polyolefin plastics, most of which are more or less opaque due principally to their partially amorphous nature. Most polyolefins do, however, have some crystallinity, and they are generally referred to as semicrystalline. High clarity is thought to be related to a low degree of crystallinity. Size and number of crystals, however, is also important. Large crystals reduce clarity, an effect generally thought to be due to diffraction and scattering of light, and most polyolefins of good clarity are predominantly microcrystalline. It is generally thought that the crystal size should be at or smaller than the wavelength of visible light to prevent light scattering, which causes opacity.
Various additives which improve the clarity of polymeric articles have been described. U.S. Pat. No. 4,016,118 proposes dibenzylidene sorbitol as an additive for improving the transparency of polyolefin articles. Mahaffey et al., in U.S. Pat. No. 4,371,645, discloses dibenzylidene sorbitol additives substituted with hydroxy, methoxy, amino, nitro and halogen groups having greater clarity enhancing effects than the parent unsubstituted compound.
In U.S. Patent application Ser. No. 116,830, filed on Nov. 3, 1987, now U.S. Pat. No. 4,845,137, and of common assignee with the present invention, alkylthio dibenzylidene sorbitol derivatives having antioxidant and clarifying properties are disclosed. This application is herein incorporated by reference.
There is a need, unaddressed hitherto in the art, for a high clarity polyolefin composition capable of radiation sterilization without discoloration or degradation of mechanical properties. It is toward fulfillment of this need that the present invention is directed.